1. Field of the Invention
This invention relates to a method for producing a copolyamide which contains meta- and para-xylylene diamines as the diamine component and adipic acid as the main dicarboxylic acid component. More specifically, this invention relates to a method for producing a copolyamide, which has the above homogeneous composition, is crystalline and colorless and is excellent in heat resistance, in accordance with a direct polycondensation method.
2. Description of the Related Art
JP-B-14925/1989 (the term "JP-A" as used herein means an "unexamined published Japanese patent application) discloses a method for producing a polyamide wherein, when a dicarboxylic acid component and a diamine component are directly mixed to carry out a polycondensation reaction under normal pressure, a reaction temperature is controlled in such a manner that a reaction start temperature higher than the melting point of the dicarboxylic acid is elevated to maintain a reaction system including a starting mixture substantially in a uniform molten state to proceed with the reaction, and the temperature of the reaction system is further elevated beyond a temperature lower by 30.degree. C. than the melting point of the resulting polyamide before a conversion reaches 95 mol % so that the reaction can proceed in the homogeneous system without losing fluidity in the system.
The above production method is characterized in that the diamine is added continuously to the reaction system while the reaction system containing the formed oligoamide/polyamide is maintained in a uniform molten state, and in this method, since the reaction is conducted under normal pressure, a polycondensation reactor does not need to be a container having pressure resistance.
Moreover, this production method does not require the time needed for operations to increase and reduce pressure to be applied to the reaction system and the time to distill off water used as a solvent in the case of an aqueous solution method. Therefore, compared with a conventional polyamide production method, this method can significantly reduce the time required for polycondensation. Further, the method does not require a calorie needed for the concentration of the aqueous solution and allows an amount to be charged for each reaction to be in a large quantity, thereby making it possible to enhance productivity. Therefore, the method is an extremely advantageous method for producing polyamide.
However, when a polyamide having a high melting point which comprises as a diamine component meta/para mixed xylylene diamine containing more than 30 mol % of paraxylylene diamine is produced with this method, relatively low volatile paraxylylene diamine is caused preferentially to react in the reaction system, and a polyamide having a higher melting point and a high content of paraxylylene group is formed. As a result, the polyamide is solidified and precipitated locally in the reaction system and a uniform molten state is hardly maintained, thereby making it impossible to obtain a product having stable performance.